It's client-side, and works very well for email. It used to be that email notifications through Thunderbird waited until your next email refresh interval, but I noticed they are coming in at the same rate as the Outlook clients after the last update to Owl.

Calendaring through Lightning is incomplete and experimental.

In the "spaces" vertical toolbar on the left, Owl has added a Teams button recently, as well. It opens the Teams O365 app in a tab in Thunderbird, which I really like. I haven't had a chance to check if any meeting or audio features work there.

It's a client-side extension for Thunderbird. It works extremely well, in my experience.